The present invention relates to ring gears, particularly as used for concurrent adjustment. The invention finds particular application in conjunction with tuning quadrature whole body resonator coils in magnetic resonance imaging apparatus and will be described with particular reference thereto. However, it is to be appreciated that the invention may find further application in transmitting adjustment inputs for tuning other magnetic resonance apparatus, for adjusting other diagnostic imaging apparatus, and the like.
Conventionally, magnetic resonance imagers include a whole body radio frequency resonator. The resonator is typically a plastic cylinder of sufficient diameter to receive inner bore lining constructions, patient couch drives, and the like while still providing sufficient diameter for the patient to be passed axially therethrough. Generally, the cylinder is 60 to 70 centimeters in diameter. Radio frequency coils are mounted on the plastic cylinder for transmitting 90.degree., 180.degree. and other radio frequency pulses into a patient received in the resonator. The radio frequency coils also receive radio frequency magnetic resonance signals from the patient. The RF coils can be distributed windings but are more commonly laminated foil segments, particularly in a high field machine.
The resonance frequency of the resonator is determined in large part by the RF coil construction, particularly its inductance and capacitance. However, when a patient is introduced into the cylinder, the effective capacitance of the coil, hence its frequency, changes. To counteract the patient capacitance, adjustable tuning reactances, commonly capacitors, are mounted on the coil. By adjusting these tuning capacitances, the resonance frequency of the coil can be returned to a preselected resonance frequency. The resonator coil assembly includes symmetric foil, wire or other RF coil portions. A pair of impedance matching networks are installed between the coil portions and a ground liner. The impedance matching networks include first and second adjustable impedances, generally capacitances, which are connected directly with the coil segments. The two tuning capacitances are mounted 180.degree. apart around the coil assembly and are mechanically interconnected to be adjusted concurrently. A pair of matching reactances, commonly capacitors are each connected between one of the tuning capacitors and ground. The matching capacitances are again mounted 180.degree. apart and interconnected mechanically to be adjusted concurrently. The RF cables to the coil are connected at the junction between the matching and tuning capacitances.
Due to symmetry requirements, the RF coil and the adjustable tuning capacitors are commonly mounted symmetrically. That is, the tuning capacitors are generally disposed about 180.degree. apart oppositely on the resonance coil. Because the two tuning capacitors must be changed by precisely the same amount during an adjustment, both are connected to a common drive. The common drive has heretofore taken the form of a drive shaft mounted parallel to the axis of the resonator coil but on the outside of its structure. Other drive shafts extended from the mounting point of the variable capacitors, about two thirds of the way down the resonator cylinder, toward the same end of the cylinder. A series of toothed belts were provided for transmitting rotational input to the drive shafts of both capacitors.
In order for the toothed belts to convey the movement around about 90.degree. of the outside of the cylinder from the drive shaft to the capacitor shafts without rubbing on it, a plurality of toothed belts were provided. A first toothed belt extended from the drive shaft a fraction of the way around a cylinder where a gear assembly transferred the motion from the first toothed belt to a second. Additional gear assemblies and tooth belts were provided until the toothed belts extended from the drive shaft to both capacitor drive shafts. In addition to the gear assemblies at the ends of each belt, each belt also had an idle or tensioning arrangement along its length.
In a manually adjusted system, the matching and tuning capacitances are adjusted for each patient. In an automatically tuned system, the capacitances may be adjusted between each image for optimal performance.
During tuning, one of the capacitor pairs, e.g. the matching capacitors, are adjusted until the response is optimized. Then, the other capacitance pair, e.g. the tuning capacitances are adjusted until the response is optimized. Because the tuning and matching capacitances interact, the matching capacitances must be readjusted after the tuning capacitances have been varied. The matching and tuning capacitances are adjusted alternately until an overall optimal response is iteratively achieved.
The whole body resonator coil is mounted in the center of the magnetic resonance imager. Often, it is one of the first structures installed with numerous components of associated hardware being mounted around it. Replacing a broken belt in the matching and tuning capacitance adjusting arrangement normally required a full day to disassemble the scanner, replace the belt, recalibrate the scanner. With a manual adjustment system, such maintenance was commonly required every six to nine months. In a computer based autotune system, the computer'speed enables the number of adjustments and adjustment iterations to be increased to such an extent that expected belt life is reduced to the order of days or weeks. The full day down time to service the belts this frequently is considered unacceptable by most purchasers.
In accordance with the present invention, a new and improved capacitor adjustment system is provided which overcomes the above referenced failure problems and others.